The present invention resides in a process for producing optimized melt-textured volume samples on the basis of the high-temperature superconductor YBa.sub.2 Cu.sub.3 O.sub.7 (YBCO) for use in contact-free self-stabilizing magnetic bearings.
Such bearings generate no friction and are not subject to the wear of parts moving relative to each other since the support surfaces are not in contact with the supported surfaces during operation. Accordingly, many of the operational problems associated with bearings presently in use are eliminated. Superconductor magnetic bearings are furthermore self-stabilizing and accordingly do not require expensive electronic control equipment for this purpose. In comparison with conventional magnetic bearings, they are substantially more simple in their design and less expensive.
For the intended application, there are various requirements as to the material. With the special technique of melt structuring, a building component can be generated which is particularly suitable for use in bearings. It provides for high levitation forces and, at the same time, for a high bearing stability.
With a certain texture with large textured areas (graina size &gt;3 cm), the superconductive properties can be so influenced that the magnetic fields are frozen during operation whereby the desired properties can be easily obtained. Microstructural defects result in an effective anchoring of the magnetic flux what is known under the term pinning and provide for excellent stabilization properties of the bearing. because of the anisotrophy of the critical currents of the high temperature superconductor YBCO, the orientation of the c axis parallel to the external magnetic field is important for achieving a high levitation force.
A basic requirement for a technical application, however, has not been fulfilled at this time, that is, the possibility to manufacture the bearing components economically with acceptable reject numbers. Only if this condition is fulfilled such bearings will be commercially utilized and installed in extremely fast running rotors such as turbomolecular pumps or flywheel energy storage devices in an efficient manner.
DE 42 43 053 A1 discloses a method for the manufacture of voluminous oxide superconductors by a three-dimensional arrangement of layers REBa.sub.2 CU.sub.3 O.sub.7. RE is any rare earth element. The method comprises a plurality of extensive and tedious handling steps such as the manufacture of intermediate molding bodies and multiple heating and cooling steps, and it utilizes in addition to Y various rare earth elements some of which are substantially more rare than Y.
The method is suitable to provide good end products but is not suitable to provide for an economical automated industrial production.
EP 0 486 698 A1 also describes a method of producing single crystal pellets by a layer arrangement of RE 123 phases in a particular order as described already in DE 42 43 053 A1.
Methods of producing such superconductors exist also in the US and in Japan.
U.S. Pat. No. 4,990,493 discloses a method of producing a polycrystalline superconductor on the basis of the compound Y-123 in which the single particles are al oriented. However, even with such an orientation, polycrystalline Y-123 is not suitable for levitation applications since the critical current, herein also called "intragrain-" or "intergrain current" is limited to the individual particles. These bodies a size at best in the range of 100 .mu.m. As a result, the integral current density averaged over the whole pellet is much too small for generating a levitation as needed for practical applications.
In the USA, a gradient process is preferred (see V. Selvamanickam et al., Appl. Phys. Lett. 60(1992) 3313-3315). In this process, the superconductive material is melted in a gradient furnace and is textured. This method provides for materials with good superconductive properties which generate high levitation forces and sufficient bearing stability but an industrial production has still not become economically feasible. Altogether, the process is a very time consuming expensive and complicated process in which the following conditions need to be observed:
the temperature gradient needs to be accurately adjustable. This requires expensive furnaces with complicated electronic controls. PA1 Each time only one sample can be textured at a time with temperature gradienting. PA1 Up to now, only relatively small samples (&lt;3 cm) have been manufactured since commercially available gradienting furnaces have a relatively small-diameter oven cavity. PA1 The necessary reproducibility could not be achieved so far, that is with this melting and texturing process a relatively large amount of rejects is produced. PA1 a) preparations of the base material: providing a commercially available powder of the compounds YBa.sub.2 Cu.sub.3 O.sub.7-x, Y.sub.2 O.sub.3 and PtO.sub.2 being ground in such a way that the YBa.sub.2 Cu.sub.3 O.sub.7-x powder has a grain size in the range of d=10.+-.2 .mu.m, whereby it has a specific surface of about 1.+-.0.2m.sup.2 /g, with an oxygen content of x&lt;0.2% and a foreign phase content of &lt;1%, a carbon content of at most 2000 ppm and a transition metal content of together at most 2000 ppm, the Y.sub.2 O.sub.3 powder has a grain size of about d=4.5 .mu.m, and the PtO.sub.2 powder has a grain size of about d=60 .mu.m. PA1 b) providing an amount suitable for the desired texturing by: processing the three powders for an optimal texturing process in stoichiometric parts according to the compound YBa.sub.2 Cu.sub.3 O.sub.7-x +0.35 mol % Y.sub.2 O.sub.3 +0.1 wt % PtO.sub.2 and mixing the compounds in a ball mill for a predetermined period limited by a certain CO.sub.2 absorption, or under an inert atmosphere until a uniform mixing state has been achieved, pressing the mixed powder uni-axially into a mold and subjecting it to a high pressure of at least 300 bar to form densified blanks of a predetermined shape, covering the surfaces of the blanks with a contamination protective cover and further densifying the blanks in a cold isostatic compression step under at least 3000 bar whereby the powder particles come into close contact with one another, and PA1 c) subjecting the blanks to a temperature treatment: by heating the blanks from ambient temperature to 600.degree. C. in 1.5 hrs, then heating the blanks from 600.degree. C. to 1100.degree. C. in 1.6 hrs, and maintaining the blanks at this temperature for about 1/2 hr, then reducing the temperature at a rate of 300.degree. C./hr to a temperature of 1040.degree. C. for about 85 hrs; then reducing the temperatures at a rate of about 5.degree. C./hr from 930.degree. C. to 850.degree. C. and then increasing the cool down speed to 50.degree. C./hr until a temperature of 450-400.degree. C. is reached, then flushing the crucible with oxygen and maintaining this temperature for at least 80 hrs and finally cooling down to ambient temperature at a rate of about, 100.degree. C./hr.
The melt-powder-melt growth process (MPMG) (U.S. Pat. No. 5,395,820) favored in Japan provides relatively good materials, but is industrially unfeasible because of unachievable economical conditions. In this Japanese process, compressed blanks are first melted at very high temperatures (about 1400.degree. C.) and are quenched before the actual melt texturing process begins. This requires a labor intensive manufacturing step for which expensive equipment for the quenching of the about 1400.degree. C. hot melt is needed. At this point, it is not clear how this process can be automated.
Accordingly, it is the object of the present invention to provide a method, whereby high temperature superconductor materials of the composition given above can be manufactured by an automated process in large numbers in an economical manner.